The present invention relates to synthetic aperture radar systems, and more particularly, to a signal processing method for use with a single-antenna multiple-pass interferometric synthetic aperture radar that mitigates atmospheric propagation error.
Prior art relating to the present invention includes traditional surveying methods, including on the ground survey with chain and transit, stereoscopic aerial photography, stereoscopic synthetic aperture radar (SAR) imaging, and more recent developments including single pass, dual channel interferometric synthetic aperture radar (IFSAR) and dual pass, single channel IFSAR.
A previous IFSAR developed by the assignee of the present invention is disclosed in U.S. patent application Ser. No. 08/249,762 filed May 26, 1994, entitled xe2x80x9cMulti-Pass and Multi-Channel Interferometric Synthetic Aperture Radars.xe2x80x9d The present invention is an augmentation and improvement to the IFSAR disclosed in this patent application. This invention operates a single channel SAR interferometer in a multiple-pass mode to achieve accuracy unobtainable by a two-pass interferometer. The terrain elevation measurement algorithm employed in this invention was optimized for low variance spatially uncorrelated phase errors typical of thermal noise. An additional multiple channel simultaneous interferometer configuration was also disclosed in this patent application.
Other related inventions of the assignee of the present invention address two or more passes of a two channel interferometer and are disclosed in U.S. Pat. No. 5,463,3978 entitled xe2x80x9cHyper-Precision SAR Interferometry Using a Dual-Antenna Multi-Pass SAR System. An atmospheric correction method for use in a two-channel interferometer is disclosed in U.S. Pat. No. 5,726,656, issued Mar. 10, 1998, entitled xe2x80x9cAn Atmospheric Correction Method for Interferometric Synthetic Array Radar Systems Operating at Long Ranges.xe2x80x9d The xe2x80x9cMulti-Pass and Multi-Channel Interferometric Synthetic Aperture Radarsxe2x80x9d invention as well as conventional two-pass interferometry, such as is disclosed by F. Li and R. Goldstein, xe2x80x9cStudies of Multi-baseline Spaceborne Interferometric Synthetic Aperture Radars,xe2x80x9d IEEE Trans. Geocsience and Remote Sensing, vol. 28, pp. 88-97, January 1990, suffers from accuracy limitations at very long ranges because of atmospheric inhomogeneities that change significantly during the time between the multiple passes.
In a single pass, dual channel interferometric synthetic aperture radar, one aircraft (or spacecraft) with two antennas collects SAR data from both antennas as disclosed by L. C. Graham, xe2x80x9cSynthetic Interferometer Radar for Topographic Mappingxe2x80x9d, Proc. IEEE, Vol. 64, pp. 763-768, June 1974. While the single pass system is convenient it is often limited in sensitivity, especially at long ranges, due to the small difference in the grazing angles for each channel.
In a dual pass, single channel interferometric synthetic aperture radar, two passes are used to collect two SAR images. The grazing angle difference is under the control of the radar operators but with increased sensitivity comes increased ambiguity far beyond that of the single-pass dual-channel approach. The accuracy of this technique suffers because of phase errors due to time-varying atmospheric inhomogeneities.
Accordingly, it is an objective of the present invention to provide for a signal processing method that may be used with single-antenna multiple-pass interferometric synthetic aperture radars that mitigates atmospheric propagation error. It is a further objective of the present invention to provide for a signal processing method for use with single-antenna multiple-pass interferometric synthetic aperture radars that improves topographic mapping accuracy at very long ranges and, as a byproduct, provides an atmospheric turbulence measurement map.
To meet the above and other objectives, the present invention provides for signal processing method useful in single-antenna multiple-pass interferometric synthetic aperture radars. The signal processing methods compute an initial elevation estimate from the phase difference between a pair of images with a relatively small elevation angle difference (i.e., a short interferometric baseline) and uses it to initialize the elevation estimation process for pairs of images with longer interferometric baselines.
This processing allows the use of a larger elevation angle difference than would have previously be feasible so that a given level of phase error due to atmospheric inhomogeneities results in proportionately less terrain elevation measurement error. The residual phase between the long- and short-baseline interferometry data, suitably filtered, is attributable to the difference in effective atmospheric propagation paths along the line-of-sight of the radar between the times that the sequence of images were taken.
The present invention is optimized for atmospheric errors exhibiting high phase variance but long spatial correlation lengths. It may be applied as a modification of the previous processing method disclosed in U.S. patent application Ser. No. 08/249,762.